Various alcoholic drinks, but particularly wine, once produced and bottled, may undergo further processes. Subsequent positive processes during ageing impart more interesting sensory features, especially for high-quality or very-high-quality wines. However, there are also negative processes, which lead to impair the wine quality.
Oxidation reactions take place in the bottle in the presence of small or very small amounts of air, normally in the presence of iron and/or copper ions. Such reactions cause compounds responsible for the organoleptic qualities of a wine, such as flavours, pigments and tannins, to degrade. It is suitable for the wine to be stabilised with respect to these oxidation reactions. Up until today, the most effective strategy has been that of reducing or removing metal content in wine. In this sense ferros cyanidation is highly effective; however, such process is extremely delicate, since too small an amount causes the wine not to be stabilised and too high an amount, with the resulting presence of deposits, leads to health damages. For these reasons, such practice is now virtually abandoned and has been almost entirely replaced by a series of measures in the manufacturing stages, aimed at avoiding contact with metals or at the use of coadjuvants, such as caseinate, which adsorb and remove part of the metals. In any case, the procedures are less effective than ferros cyanidation.
Another fundamental aspect of wine is the colour thereof. In red wine the colour is due to a series of phenol complexes, found in wine and coming mostly from the grapes skins. Over time, such complexes tend to polymerise and to form a precipitate. Such drawback today is faced with specific tub procedures, such as microoxigenation, ageing, clarification, refrigeration and others. In other cases gum arabic is added.
However, the most common instability seems to be the one due to potassium bitartrate, generically called tartaric instability.
At the wine pH, the naturally occurring tartaric acid in wine is dissociated in various forms; the prevailing one is hydrogen-tartrate or bitartrate (HT−) which reacts with potassium, itself contained abundantly in wine. The tartrate ion (T2−) reacts instead with bivalent cations, in particular with calcium.
Wine has a concentration of these anions often exceeding the solubility product thereof, which determines a state of oversaturation. A meta-stable situation hence occurs, in which there is no tartrate crystal forming and precipitation due to the simultaneous presence of other agents which act as protective colloids. However, also a slight change of the environmental conditions, for example a temperature drop during transportation and storage, can change the equilibrium and lead to crystallisation phenomena in the bottle, with the resulting formation of precipitates.
Instability with regard to these phenomena is measured with various tests. The most common one is the electric conductivity drop, measured in ΔμS/cm. The smaller this value, the more stable to tartaric precipitation the wine.
So far various strategies exist to reduce to a minimum this risk. In certain cases, before bottling, wine is brought to a temperature below 0° C., possibly adding bitartrate crystals, so that precipitation occurs before bottling and the wine can be bottled after filtering. Although conceptually simple and widely used, however, this strategy has the big drawbacks of employing large amounts of energy to reduce and keep the temperature low, of considerably extending processing times and of requiring sprcific equipment which increases the investments necessary for the management of a wine cellar.
Another strategy is that of removing from the wine part of the excess cations; the preferred technique to actuate this strategy is electrodialysis, which, however, also implies costly and delicate equipment, difficult to be available for small manufacturers, and which implies high water consumption and the creation of large amounts of wastewater.
Finally, the last strategy consists in adding crystallisation inhibitors. This path is preferable to others, since it has no adverse environmental effects and can be applied also in very small companies. From as far back as the '50s, the use of metatartaric acid has been suggested and adopted, a substance obtained through esterification by thermal processing of tartaric acid, capable of inhibiting the formation of bitartrate crystals. The limit of metatartaric acid, which is inexpensive and easily employable, is the short duration of the inhibiting effect, which tends to disappear after a few months, especially following accidental heating of the wine.
In order to inhibit crystallisation, mannoproteins with specific molecular weight are employed, extracted from yeast, as proposed by Maine and Dubordieu as well as by Lankhorste et al.
The limit of this technique is the high cost of the product and the variableness of results in certain wine types.
Another inhibitor recently authorised in Europe is carboxyimethylcellulose (CMC), a polymer obtained by chemically treating cellulose. It is inexpensive, but in red wines it often causes colour precipitation, in addition to giving sporadic filtering problems and to being little effective in highly unstable wines.
Gum arabic is another tartaric stabiliser used before bottling, even though with a gentler effect and, individually, not sufficient to avoid precipitations in the most critical wines.
US 2010/0119671 A1 discloses a composition for wine stabilising against the precipitation of tartaric acid salts, comprising at least 2.5% of a mixture of peptides having a molecular weight ranging between 3,000 and 10,000.